Thermal treatment of recycled compacted concrete: Effects on physical properties and hydration characteristics

Abstract

High-pressure compaction is a novel approach for converting waste concrete into reusable concrete without additional materials. Optimal thermal treatment of recycled compacted concrete (also referred to as “compact”) results in lower carbon emissions compared with cement-based materials and increased strength. In this study, to realize the application of thermal treatment in compaction recycling, we investigated its effects on the physical and hydration characteristics of waste concrete, which are not yet fully understood. To achieve this, an investigation was conducted on thermally treated (heating in oven or in autoclave) compacts prepared from ordinary concrete powder (≤300 μm) by replacing up to 100% of natural fine and coarse aggregates with slag sand and slag gravel. Loss-on-ignition in a furnace and thermogravimetric analysis were used to measure the degree of hydration. Moreover, the effects of the water saturation and particle size (1.18 mm) on the compressive strength of the compact were investigated. The results indicated that a larger amount of slag resulted in improved physical and mechanical properties of the compact under both untreated and heat-treated conditions. Furthermore, 50% inclusion of large particles (1.18 mm) after autoclaving satisfied the normal-strength criteria as per Japanese Standards (JIS A 5308 “Ready-mixed concrete”). An in-depth analysis of the hydration behavior of the compacts indicated that sufficient hydration (up to 92%) occurred after thermal treatment, although the compacts were made of waste concrete, suggesting that the hardened cement in waste concrete can be thermally activated. This study provides novel insights into the relationship between the physical and hydration characteristics and strengths of compacts—a topic that has received limited attention in the literature. The findings suggest that thermally treated compacts are viable materials for prefabricated nonstructural applications, such as the production of pavement blocks and bricks.